The circular field defining apertures commonly used in isocentrically focussed high-dose irradiation (termed radiosurgery) used in treatment of surgically inaccessible vascular malformations, benign acoustic neuromas, or other focal neoplasms, are inadequate to deliver a uniform dose to non-spherical lesions and to simultaneously minimize dose to surrounding critical brain tissue. Multiple-isocenter techniques introduce unacceptable dose Inhomogeneity within the target volume and dose inhomogeneity has been identified as the major predictor of complications following stereotactic radiosurgery (SRS). There is clearly a need for improved techniques. Evaluation of lesion shapes In 40 previously treated patients showed that in nearly 2/3 of the cases, dose to normal tissue could have been reduced and target dose uniformity improved by using dynamically shaped irradiation fields continuously conforming to the target shape throughout the treatment-arc sequence. A computer-controlled Dynamic Field Shaping Collimator for small-field radiosurgery is proposed. Four independent jaws, each having translational motion (in-out) and circular rotation (around the target-to-isocenter-axis) modify the field shape of the circular collimator to conform the shape more closely to the target projection at each increment of arc. The mechanical integrity and reproducibility of the collimation system will be investigated. The dosimetric characteristics of the shaped fields will be Investigated by film and water phantom dosimetry. Dose calculation techniques will be developed and verified to correctly predict the dose distribution for each possible arrangement of the four independent jaws and the series of circular-aperture inserts. These calculations will be verified both for fixed-field and arc irradiations using head phantoms and film densitometry. Computer calculation of isodose distributions and dose volume histograms will be used to compare radiosurgery treatment techniques using a single aperture and single isocenter, single or multiple apertures with multiple Isocenters, and single aperture with dynamic field shaping. It is expected, based on simple geometric estimations, that the volume of critical normal brain receiving excessive radiation can be reduced by half. The potential benefit of this work includes improved cure rates and reduced acute and long term complications through Increased dose uniformity throughout the target volume and minimization of dose to critical brain tissue. These techniques may be applicable in 1/2 to 2/3 of all patients treated with SRS.